(1) Field of the Invention
The present invention relates to an adhesive tape having transversely tearable property which is used for packaging and the like purposes. More particularly, the present invention relates to an adhesive tape having transversely tearable property which is prepared by using a nonwoven fabric stretched mainly in a longitudinal direction that is made by spinning a thermoplastic resin as its longitudinal base material, and by laminating the same with a transverse base material, an adhesive layer and a release layer.
It is to be noted that the term "longitudinal base material" used herein means the orienting direction of fibers contained in the material extends in the longitudinal direction of the resulting adhesive tape, and the term "transverse base material" means the orienting direction of fibers in the material extends in the direction perpendicular to the longitudinal direction of the adhesive tape.
(2) Prior Art
In the field of adhesive tape, a woven fabric of spun yarns mainly made of rayon staple fiber or cotton has been hitherto employed. There are, however, such problems that spun yarn is not strong and that, in a wet condition, the spun yarn is swollen or low in strength. In addition, it has been difficult to produce hitherto demanded thin adhesive tapes. Furthermore, the fluctuations in the market prices of rayon staple fiber and cotton are large in an economical viewpoint, so that it has not been suitable as an industrial raw material which is required for the stable supply in the case of adhesive tape. In this respect, a base cloth for pressure sensitive adhesive tape prepared from vinylon or polyester long fiber filaments has been investigated. However, because the woven fabric made of these synthetic fibers have poor transversely tearable property, it is required to densely weave thin yarns for avoiding such disadvantages as described above (Japanese Laid-open Patent Publication Nos. Sho 58-91845, Sho 60-71735, Sho 63-306037, and Hei 5-44135, etc.), so that the weaving cost therefor increases, and the resulting product is poor in the quality and the cost efficiency. For these reasons, the woven fabric of spun yarns made of rayon staple fiber could not have sufficiently been replaced by the adhesive base cloth made of such synthetic fiber as described above. There is an adhesive base cloth likewise made of polyolefin flat yarns as a raw material (Japanese Laid-open Utility Model Publication No. Hei 2-3821, etc.). However, because the resulting base cloth has poor transversely tearable property, it is required to densely weave thin yarns for eliminating the disadvantage described above, so that the weaving cost therefor increases and the resulting product is also poor in the quality and the cost efficiency, besides this product involves another disadvantage of low heat resistance because the raw material is a polyolefin.
In the meantime, with the recent development in automated machines such as automatic packaging machines, the operation speed of them are increased, so that the frequency for changing adhesive tapes to be used becomes over several times in an hour. Because this is very troublesome, it has been required to increase the roll length of an adhesive tape by reducing the thickness thereof while maintaining the same diameter of a roll. In this respect, however, it was difficult to reduce the thickness of such an adhesive tape, because the yarns contained in a woven fabric are bent.
The present inventors have developed and industrialized various types of cross-laminated nonwoven fabric which is prepared by cross-laminating a longitudinally aligned nonwoven fabric having high strength in its longitudinal direction with a transversely aligned nonwoven fabric having high strength in its transverse direction (Japanese Patent Publication Nos. Sho 49-48580, Sho 50-40185, Sho 53-38783, Sho 55-51058, Sho 57-30368, Sho 57-54581, Sho 62-28226, Hei 1-60408, and Hei 3-36948 etc.). The application of these cross-laminated nonwoven fabrics for an adhesive tape has been tried (Japanese Laid-open Patent Publication Nos. Sho 58-106737 and Hei 1-204983, etc.). In accordance with the present invention, the above-mentioned projects have been further improved and developed.
A cross-laminated nonwoven fabric is manufactured in accordance with such a manner wherein a longitudinally aligned nonwoven fabric is cross-laminated with a transversely aligned nonwoven fabric, and the productivity thereof ranges from 40 m/min. to 100 m/min. On the other hand, it is necessary for utilizing thin yarns to prepare a dense woven fabric for the sake of increasing the transversely tearable property in a base cloth for adhesive tape. Accordingly, it is difficult to attain the productivity of 1 m/min. or higher in the latter case, even if a recently improved weaving machine is employed. In other words, the productivity in the cross-laminated nonwoven fabric according to the present invention is higher than that of a woven fabric for cloth adhesive tape by around 100 times.
Moreover, the transversely tearable property is obtained by a tearing mechanism wherein tear stress propagates effectively in the transverse direction of a tape. For this reason, it is required to densely weave the woven fabric in such a degree that the looseness of the texture is lost. However, a densely woven fabric contains bent yarns so that such woven fabric does not efficiently exhibits the strength of filaments. Accordingly, about 70 to 80% of the strength of original yarn is merely utilized in such case as described above. On the contrary, because the filaments are not bent in a cross-laminated nonwoven fabric, the strength of the filaments composing the nonwoven fabric is demonstrated sufficiently. Furthermore, because the strength of filaments contained in a cross-laminated nonwoven fabric is efficiently utilized, the amount of the filaments to be used becomes comparatively small in comparison with that of a densely woven fabric. In addition, because the filaments are not bent in a cross-laminated nonwoven fabric, the thickness of obtained filament layer is small, as a result, it is possible to provide a thin adhesive tape.
While a cross-laminated nonwoven fabric has the characteristic features as described above, the longitudinally aligned nonwoven fabric contained therein prevents the effective transverse propagation of transverse tear stress, because the longitudinally aligned nonwoven fabric exists independently from the transversely aligned nonwoven fabric in the case that the cross-laminated nonwoven fabric is used as the base cloth for adhesive tape. As described above, it is necessary to solve the problem of transversely tearable property due to the structure peculiar to stretched nonwoven fabric.
Then, it is the same in either case of woven fabric or stretched nonwoven fabric, there is a need for processing such woven or nonwoven fabric with an adhesive dissolved in an organic solvent in order to enhance the adhesive property relative to an extrusion laminated polyethylene which is used as the release layer in most cases in the event where rayon staple fiber or polyester is used for the base material, which processing is called as "anchoring treatment". An example of the agent for the anchoring treatment includes generally a toluene solution of an alkyl titanate and the like, and other primers such as epoxy type, acrylic type, polyethylene imine type, and isocyanate type primers may also be used.
The anchoring treatment, however, requires not only an extra cost for adhesive, but also an extra equipment and work site for effectively removing the exhaust gas of the organic solvent in view of the controlling of environmental pollution. Moreover, if any trouble is caused to occur in such a processing equipment, there remains many problems to be solved in the cost, safety, and the control for environmental pollution. It is also the subject matter of the present invention to provide the adhesive tape which can be produced without using such an anchoring agent.